The form of a Brillouin gain spectrum (hereinafter referred to as BGS) of Brillouin scattered light outputted from an optical fiber changes depending on the temperature and strain of the optical fiber. There is a technique which measures the temperature and strain by utilizing this change.
Known as an example of the technique for measuring the BGS of Brillouin scattered light is BOCDA (Brillouin Optical Correlation Domain Analysis) (see Nonpatent Document 1). In the BOCDA, probe light and pumping light modulated with the same modulation frequency and modulation index are made incident on an optical fiber opposingly from different end parts thereof. Here, while sweeping the difference (hereinafter referred to as optical frequency difference) between the respective center frequencies of probe light and pumping light, the BOCDA measures the BGS of Brillouin scattered light occurring in a place exhibiting a correlation peak.
The BGS occurs in a search domain depending on the modulation frequency and modulation index of the probe light and pumping light. The location of the search domain depends on the phase difference between the probe light and pumping light.
Non-patent Document: Kazuo HOTATE, et al., “Simplified System of Fiber Brillouin Optical Correlation Domain Analysis for Distributed Strain Sensing”, 16th International Conference on Optical Fiber Sensors (OFS-16), We2-3, p. 290-293, October 2003.